The aim of this investigation is to identify, solubilize, purify and characterize the phosphate carrier of mammalian mitochondria. Unpublished research indicates that the phosphate carrier may be identified among the proteins of beef heart or rat heart mitochondrial inner membranes by selective labeling with radioactive N-ethyl maleimide (NEM), followed by sodium dodecylsulfate (SDS) gel electrophoresis or by chromatography on Sephadex in the presence of SDS. Using the information gained on the properties of the NEM-labeled protein, experiments are planned to isolate the uninhibited P(i) carrier by SDS-gel filtration and/or byaffinity chromatography and to document the phosphate-binding capacity of the isolated carrier.